1. Field of the Invention
The present invention relates to an image heating apparatus suitable as a heat-fixing apparatus mounted in an image forming apparatus such as a copying machine or a printer and, more particularly, to an image heating apparatus of electromagnetic induction heating type.
2. Related Background Art
A heating apparatus of electromagnetic induction heating type produces heat from an eddy current that is generated by placing an electroconductive member (e.g., an electromagnetic induction heat generating member, an inductive magnetic material, or an electroconductive material absorptive of magnetic fields) in a magnetic field. For example, this type of heating apparatus is effective, in an electrophotographic, electrostatic recording, magnetic recording, or other image forming apparatus, as a heat-fixing apparatus for heating a recording medium that carries an unfixed toner image and thus obtaining a permanently-fixed image through heat-fixing, or as an image heating apparatus for heating a sheet which has a porous macromolecular layer on the surface and on which an image is formed by ink-jet or other methods and thus melting the porous macromolecular layer for surface treatment.
In most image heating apparatuses of electromagnetic induction heating type, an excitation coil that serves as magnetic field generating means is placed inside a heating roller that is constituted of an electroconductive member (see Japanese Patent Application Laid-Open No. 2000-275991, for example).
FIG. 10 is a perspective view of an air-core coil used in a fixing apparatus that is disclosed in Japanese Patent Application Laid-Open No. 2000-275991.
In the prior art example given above, an excitation coil 2 that serves as magnetic field generating means is placed inside a heating roller, which is an electroconductive member.
The excitation coil 2 is a litz wire wound several times, and the litz wire is obtained by twisting together plural strands which are copper wires covered with polyamideimide. The litz wire is placed to line the inner wall of the fixing roller (heating roller) covering, on one side, about a 150° area out of the 360° central angle of the circular sectional shape of the fixing roller, and a twice larger area on both sides, namely, about a 300° area.
To manufacture this type of cylindrical coil whose litz wire covers most of the area in the circumferential direction of a fixing roller, a cylindrical winding jig is necessary and the litz wire is wound around the jig.
However, it is difficult to remove the coil from the jig if the coil actually covers the 300° area.
For that reason, the coil in the prior art example given above is composed of two parts which are semi-cylindrical coils formed separately and then connected to each other. The strands of one half coil are together connected to the strands of the other half coil at a single point in a junction portion 21, instead of connecting the strands one by one. In this way, work processes in the junction portion are reduced in number and secure connection is obtained.
With this structure, the winding jig can readily be removed after the litz wire is wound around the jig.
On the other hand, inserting the coil 2 into the fixing roller is not so easy since the strands of the coil are usually composed of copper wires, which lack rigidity. Although insertion of the coil into the fixing roller could be facilitated by attaching the coil to a holder, there is still a problem and the coil could be damaged if the coil bumps against the fixing roller. Furthermore, it is difficult to set the nonrigid coil in place while ensuring a desired distance between the coil and the fixing roller along the length of the fixing roller.
When the coil and the fixing roller are not spaced apart from each other as designed, temperature distribution on the fixing roller becomes uneven causing a failure in fixing toner in an electrophotographic process and uneven sheet surface treatment in an ink-jet process. In addition, the coil 2 obtained by joining two halves at the junction portion 21 is poor in mass-producibility and is difficult to handle.